The synaptic organization of the prepacemaker nucleus in weakly electric knifefish, Eigenmannia: a quantitative ultrastructural study

Walter Heiligenberg: the jamming avoidance response and beyond

Walter Heiligenberg (1938-1994) was an exceptionally gifted behavioral physiologist who made enormous contributions to the analysis of behavior and to our understanding of how the brain initiates and controls species-typical behavioral patterns. He was distinguished by his rigorous analytical approach used in both behavioral studies and neuroethological investigations. Among his most significant contributions to neuroethology are a detailed analysis of the computational rules governing the jamming avoidance response in weakly electric fish and the elucidation of the principal neural pathway involved in neural control of this behavior. Based on his work, the jamming avoidance response is perhaps the best-understood vertebrate behavior pattern in terms of the underlying neural substrate. In addition to this pioneering work, Heiligenberg stimulated research in a significant number of other areas of ethology and neuroethology, including: the quantitative assessment of aggressivity in cichlid fish; the ethological analysis of the stimulus-response relationship in the chirping behavior of crickets; the exploration of the neural and endocrine basis of communicatory behavior in weakly electric fish; the study of cellular mechanisms of neuronal plasticity in the adult fish brain; and the phylogenetic analysis of electric fishes using a combination of morphology, electrophysiology, and mitochondrial sequence data.

Neurogenesis, cell death and regeneration in the adult gymnotiform brain

Gymnotiform fish, like all teleosts examined thus far, are distinguished by their enormous potential for the production of new neurons in the adult brain. In Apteronotus leptorhynchus, on average 10(5) cells, corresponding to approximately 0.2 % of the total population of cells in the adult brain, are in S-phase within any period of 2 h. At least a portion of these newly generated cells survive for the rest of the fish's life. This long-term survival, together with the persistent generation of new cells, leads to a continuous growth of the brain during adulthood. Zones of high proliferative activity are typically located at or near the surface of the ventricular, paraventricular and cisternal systems. In the central posterior/ prepacemaker nucleus, for example, new cells are generated, at very high rates, in areas near the wall of the third ventricle. At least some of these cells differentiate into neurons, express immunoreactivity against the neuropeptide somatostatin and migrate into more lateral areas of this complex. Approximately 75 % of all new brain cells are generated in the cerebellum. In the corpus cerebelli and the valvula cerebelli, they are produced in the molecular layers, whereas in the eminentia granularis the newborn cells stem from proliferation zones in the pars medialis. Within the first few days of their life, these cells migrate towards specific target areas, namely the associated granule cell layers. At least some of them develop into granule neurons. The high proliferative activity is counterbalanced by apoptosis, a mechanism that resembles the processes known from embryonic development of the vertebrate brain. Apoptosis also appears to be used as an efficient mechanism for the removal of cells damaged through injury in the brain of adult Apteronotus leptorhynchus. Since apoptosis is not accompanied by the side effects known from necrosis, this ‘clean’ type of cell death may, together with the enormous proliferative activity in the brain, explain, at least partially, the tremendous capability of teleost fish to replace damaged neurons with newly generated ones. One factor that appears to play a major role in the generation of new cells and in their further development is the neuropeptide somatostatin. In the caudal cerebellum of the gymnotiform brain, somatostatin-binding sites are expressed, at extremely high densities, at sites corresponding to the areas of origin, migration and differentiation of the newborn cells. This pattern of expression resembles the expression pattern in the rat cerebellum, where somatostatin immunoreactivity and somatostatin-binding sites are transiently expressed at the time when the granule cells of the cerebellum are generated. Moreover, after mechanical lesions of the corpus cerebelli, the expression of somatostatin-like immunoreactivity is tremendously increased in several cell types (presumably astrocytes, microglia and granule cell neurons) near the path of the lesion; the time course of this expression coincides with the temporal pattern underlying the recruitment of new cells incorporated at the site of the lesion.

Neural circuitry for communication and jamming avoidance in gymnotiform electric fish

Over the past decade, research on the neural basis of communication and jamming avoidance in gymnotiform electric fish has concentrated on comparative studies of the premotor control of these behaviors, on the sensory processing of communication signals and on their control through the endocrine system, and tackled the question of the degree to which these behaviors share neural elements in the sensory-motor command chain by which they are controlled. From this wealth of investigations, we learned, first, how several segregated premotor pathways controlling a single central pattern generator, the medullary pacemaker nucleus, can provide a large repertoire of behaviorally relevant motor patterns. The results suggest that even small evolutionary modifications in the premotor circuitry can yield extensive changes in the behavioral output. Second, we have gained some insight into the concerted action of the brainstem, the diencephalon and the long-neglected forebrain in sensory processing and premotor control of communication behavior. Finally, these studies shed some light on the behavioral significance of multiple sensory brain maps in the electrosensory lateral line lobe that long have been a mystery. From these latter findings, it is tempting to interpret the information processing in the electrosensory system as a first step in the evolution towards the ‘distributed hierarchical’ organization commonly realized in sensory systems of higher vertebrates.

An in vitro technique for tracing neuronal connections in the teleost brain

The availability of neuronal tract-tracing techniques has been fundamental to the development of the neurosciences. While most of the previously described methods are performed in vivo, in the present paper, detailed protocols are reported for tracing neuronal connections in an in vitro preparation. This technique, tested in various neural systems of the teleost brain, allows precise application of tracer substance(s) under visual control. After the isolation of the brain, the tissue is kept alive by superfusion with oxygenated artificial cerebrospinal fluid in a slice chamber. Neuronal connections are traced by the application of crystals of biocytin or dextran-tetramethylrhodamine to the region of interest. Following intracellular transport over 8-18 h, the tissue is fixed and processed histochemically for visualization of structures filled with the tracer substance. This method can readily be modified for double labelling. Step-by-step procedures are outlined for (a) the simultaneous detection of two tracer substances in the same tissue sample, (b) the combination of tract tracing with the immunohistochemical identification of various biochemical markers such as 'classical' transmitters and neuropeptides, and (c) the visualization of both traced structures and mitotically active cells labelled with the thymidine analogue 5-bromo-2'-deoxyuridine. By exhibiting a high degree of efficiency, the described in vitro tract-tracing technique represents also a significant contribution towards a reduction of living animals in neurobiological experimentation.

Reciprocal connections between the preglomerular nucleus and the central posterior/prepacemaker nucleus in the diencephalon of weakly electric fish, Apteronotus leptorhynchus

The central posterior/prepacemaker nucleus of gymnotiform fish is a bilateral cell group located in the dorsal thalamus. This complex consists of approximately 10,000 neurons which can be divided into several subpopulations. One subpopulation comprised of a few hundreds of neurons projects to the pacemaker nucleus in the medulla oblongata, thus constituting the prepacemaker nucleus portion of this complex. By employing in vitro tract-tracing techniques, we have, in the present investigation, examined the pattern of connectivity formed by the central posterior/prepacemaker nucleus with a diencephalic cell group, the preglomerular nucleus. As demonstrated by anterograde and retrograde tracing, a subpopulation of several hundreds of neurons located in the central posterior/prepacemaker nucleus project to the ipsi- and contralateral preglomerular nucleus. Double-labelling experiments revealed that at least a fraction of these neurons also innervate the pacemaker nucleus. In the preglomerular nucleus, a large number of neurons give rise to projections that terminate in the ipsilateral central posterior/prepacemaker nucleus. The reciprocal connection between the central posterior/prepacemaker nucleus and the preglomerular nucleus may be used to relay sensory information directly conveyed to one of the two nuclei indirectly to the other nucleus. The existence of at least some central posterior/prepacemaker nucleus neurons projecting to both the preglomerular nucleus and the pacemaker nucleus may provide the morphological basis for the transmission of an efference copy of electromotor information produced by neurons in the central posterior/prepacemaker nucleus to the preglomerular nucleus.

The postembryonic development of somatostatin immunoreactivity in the central posterior/prepacemaker nucleus of weakly electric fish, Apteronotus leptorhynchus: a double-labelling study

The neuropeptide somatostatin (SS) is widely distributed in both the central and peripheral nervous system of vertebrates. Its widespread distribution is paralleled by a large variety of diverse functions. While embryonic and perinatal development of SS-like immunoreactivity have been well examined, little is known about the postnatal development of this neuropeptide. Since, in teleosts, neurogenesis persists in many brain regions during adulthood, these vertebrates are well suited to investigate this phenomenon. In the present study, we have, therefore, examined the development of somatostatinergic cells born during adulthood in the central posterior/prepacemaker nucleus (CP/PPn) of Apteronotus leptorhynchus, a weakly electric gymnotiform fish. This was achieved by labelling proliferating cells with the thymidine analogue 5-bromo-2'-deoxyuridine (BrdU) and by simultaneous immunocytochemical detection of SS-like immunoreactivity. SS-like immunoreactivity is adopted in a period between 2 days and 3.5 days after birth. While the number of BrdU-labelled cells in the CP/PPn decreases 10 days after birth, the percentage of double-labelled cells among the BrdU-labelled cells remains with 1.0-7.6% in the period between 3.5 days and 100 days after birth rather constant. This percentage matches well the fraction of SS-positive cells in the total population of cells present in the CP/PPn.

Peptidergic transmission: from morphological correlates to functional implications

Like non-peptidergic transmitters, neuropeptides and their receptors display a wide distribution in specific cell types of the nervous system. The peptides are synthesized, typically as part of a larger precursor molecule, on the rough endoplasmic reticulum in the cell body. In the trans-Golgi network, they are sorted to the regulated secretory pathway, packaged into so-called large dense-core vesicles, and concentrated. Large dense-core vesicles are preferentially located at sites distant from active zones of synapses. Exocytosis may occur not only at synaptic specializations in axonal terminals but frequently also at nonsynaptic release sites throughout the neuron. Large dense-core vesicles are distinguished from small, clear synaptic vesicles, which contain "classical' transmitters, by their morphological appearance and, partially, their biochemical composition, the mode of stimulation required for release, the type of calcium channels involved in the exocytotic process, and the time course of recovery after stimulation. The frequently observed "diffuse' release of neuropeptides and their occurrence also in areas distant to release sites is paralleled by the existence of pronounced peptide-peptide receptor mismatches found at the light microscopic and ultrastructural level. Coexistence of neuropeptides with other peptidergic and non-peptidergic substances within the same neuron or even within the same vesicle has been established for numerous neuronal systems. In addition to exerting excitatory and inhibitory transmitter-like effects and modulating the release of other neuroactive substances in the nervous system, several neuropeptides are involved in the regulation of neuronal development.

Apoptosis as a regulator of cell proliferation in the central posterior/prepacemaker nucleus of adult gymnotiform fish, Apteronotus leptorhynchus

Like many species of teleost fish, the gymnotiform Apteronotus leptorhynchus displays a high degree of proliferative activity in a large number of brain regions during adulthood. One of these regions is the central posterior/prepacemaker nucleus (CP/PPn) in the diencephalon. By applying in situ techniques for the detection of DNA fragmentation, a feature characteristic of apoptotic cells, we show in the present study that the high proliferative activity in the CP/PPn is counterbalanced by programmed cell death. Most of the apoptotic events occur in the ventricular and subventricular zones of this thalamic complex, where the generation of the cells and their differentiation into neurons take place. The demonstration of apoptosis in the CP/PPn provides strong evidence against the hypothesis that animals in which neurogenesis continues beyond embryonic stages of development lack cell death.

Somatostatin in the prepacemaker nucleus of weakly electric fish, Apteronotus leptorhynchus: evidence for a nonsynaptic function

Neuropeptides are widely distributed throughout the nervous system and exert a large number of heterogeneous functions. While they are synthesized in the soma, release is thought to take place in axonal terminals of neurons. A good model system to investigate the role of peptides in the nervous system is provided by the central posterior/prepacemaker nucleus (CP/PPn) of pacemaker nucleus (Pn), a medullary cell group controlling the electric organ discharge (EOD). Previous immunocytochemical and in situ-hybridization studies employing topographical criteria indicated that PPn neurons may express the neuropeptide somatostatin (SS). In the present study, we unambiguously identified PPn neurons by in vitro tract tracing. By combining this technique with SS immunocytochemistry, we found that a large portion of retrogradely labelled PPn neurons exhibited SS-like immunoreactivity (72-89%, n = 708 cells in 10 fish examined). Surprisingly, however, neither the proximal PPn axons nor anterogradely labelled terminals innervating the Pn displayed significant amounts of SS-like immunolabelling (n = 10 fish examined in each experiment). These results and the lack of SS binding sites in the Pn [82] suggest that SS expressed by PPn cells is not synaptically released at the target site of their axons, the Pn, but acts via a nonsynaptic mechanism in the CP/PPn proper.

Birth and migration of neurons in the central posterior/prepacemaker nucleus during adulthood in weakly electric knifefish (Eigenmannia sp.)

In contrast to mammals, fish maintain their capacity to generate neurons in the central nervous system even during adulthood for prolonged periods of life. By employing immunohistochemical, autoradiographic, and electron microscopic techniques, we studied such a postnatal neurogenesis within the complex of the central posterior/prepacemaker nucleus (CP/PPn) in knifefish (Eigenmannia sp.), a weakly electric teleost. The CP/PPn is a bilateral cluster of neurons in the thalamus. It controls frequency modulations of the electric organ discharge as they are used during social interactions. In the CP/PPn region adjacent to the wall of the third ventricle ("ventricular zone"), cells are born continuously and at high rates. They undergo multiple cell divisions before differentiating into neurons. Concomitant with this development, the newborn neurons migrate toward lateral regions of the CP/PPn. In the course of this lateral migration, they appear to acquire immunological and morphological characteristics that are typical for mature CP/PPn neurons. We hypothesize that at least some of the newly generated cells develop finally into functional CP/PPn neurons.

The structure of the diencephalic prepacemaker nucleus revisited: light microscopic and ultrastructural studies

The prepacemaker nucleus (PPn), a bilateral cluster of neurons at the boundary of diencephalon and mesencephalon, controls frequency modulations of the electric organ discharge in weakly electric knifefish (Eigenmannia sp.). Previous light microscopic studies employing retrograde labelling with horseradish peroxidase suggested that the PPn is restricted to a small area, located approximately 400 microns laterally from the third ventricle and fusing at its medial edge with the thalamic central posterior nucleus (CP). In the present investigation we used Phaseolus vulgaris-leucoagglutinin and cholera toxin as highly sensitive markers. In contrast to the previous studies, these experiments yielded a large number of labelled cells not only in the region of the traditionally defined PPn but also in an area reaching far into the CP. Since the PPn has been defined by retrograde labelling rather than by topographic criteria, this result questions the traditional separation between PPn and CP. Such a notion is in agreement with observations of Nissl-stained sections at the light microscopic level and with a quantitative analysis of several morphological characteristics of the cell bodies in the PPn and CP at the ultrastructural level. Both sets of experiments failed to find differences between the two nuclei. Furthermore, autoradiographic studies have shown that, even in adulthood, cells are continuously born within the ventricular zone of the CP, and at least some of these newborn cells differentiate into CP cells and migrate laterally towards the PPn. Therefore, we postulate that CP and PPn form one large complex, with the medial CP providing precursors of neurons in the lateral CP and PPn.

Somatostatin-like immunoreactivity in the region of the prepacemaker nucleus in weakly electric knifefish, Eigenmannia: a quantitative analysis

The diencephalic prepacemaker nucleus (PPn) is a bilateral cluster of neurons that controls frequency modulations of the otherwise very regular electric organ discharge cycle in weakly electric knifefish, Eigenmannia. Ultrastructural evidence had suggested that the action of excitatory and inhibitory synapses contacting PPn neurons is modulated by neuropeptides. In this investigation, we examined the distribution of somatostatin-like immunoreactive (S-IR) structures in the region of the PPn. In transverse sections, we found 5 bilateral S-IR structures in the region of the PPn: a 'dorsohorizontal stripe'; a 'lateral cell group'; a 'diagonal stripe'; a 'tubercular stripe'; and a 'hypothalamic stripe'. The dorsohorizontal stripe, consisting of S-IR fibers, terminals, and roughly 200 cell bodies unilaterally, stretches from the edge of the third ventricle at the thalamic dorsal-posterior nucleus and the central posterior nucleus approximately 400 microns laterally to the PPn. S-IR cell bodies show a distinct pattern of distribution within this stripe. Almost half of the somata are located within 50 microns of the ventricle. These ventricular cells are small and often densely clustered. Laterally, towards the PPn, the number of labelled cells decreases, whereas their size gradually increases. The lateral cell group consists of roughly 20 somata in the medial region of the subelectrosensorius nucleus. Fibers of the diagonal stripe travel from the hypothalamus dorsalis to the PPn. Fibers of the tubercular stripe originate from S-IR cell bodies in the medial zone of the periventricular nucleus of the posterior tuberculum and merge with the diagonal stripe. Fibers of the hypothalamic stripe connect the hypothalamus lateralis and the diagonal stripe. The density of immunolabelling in the hypothalamic stripe is significantly higher in mature than in immature females.

Clustering of cell bodies, bundling of dendrites, and gap junctions: morphological substrate for electrical coupling in the prepacemaker nucleus

The possible morphological basis for electrical coupling between neurons of the prepacemaker nucleus was studied in weakly electric gymnotiform fish at the ultrastructural level. Three structural characteristics were found: Extremely dense clustering of cell bodies; 'bundling' of dendrites; and gap junctions between neurons. Electrical coupling may take place through gap junctions and the spatial arrangement of elements in the prepacemaker nucleus, which could enable ephaptic interactions. Such mechanisms may also be used for averaging the responses of individual neurons in the whole assembly in order to render more predictable behavioral reactions.